In a communication system, data is sent from a transmitter, over a channel, to a receiver. For example, data may be routed from one part of the world to another through one or more networks, where each network includes network entities connected by channels. When the data is to be transmitted from a first network entity to a second network entity, a transmitter in the first network entity transmits the data to a receiver in the second network entity over the channel connecting the transmitter to the receiver. In an optical network, the channel may be an optical channel having one or more optical fibers.
As the demand for data increases, there is a desire to transmit more data over a channel. The spectral efficiency of a channel refers to the information rate that can be transmitted over a given bandwidth on the channel. It is desired to increase the data rate through a corresponding increase in spectral efficiency.
One technique to try to increase the spectral efficiency is to use a higher order modulation, which means increasing the number of bits represented by a single symbol transmitted over the channel. However, this has potential downfalls. For example, in the context of an optical communication system, using a higher order modulation often requires using a higher launch power. Higher launch powers may lead to enhanced non-linearity impairment, and so there may be a power constraint on the launch power. Using a higher order modulation may also require an analog-to-digital converter (ADC) and a digital-to-analog (DAC) converter that have a higher resolution. There may be increased noise at a high sampling rate because the DAC/ADC combination may behave like a low pass filter with limited bandwidth that tends to cut the signal and introduce inter-symbol interference. Also, transmissions using higher order modulation may be more sensitive to power imbalance in the I and Q branches and/or may be more sensitive to skew between the I and Q branches.
Another technique is to try to increase the data rate by increasing the baud rate by shortening the transmission period of each symbol. This also has potential downfalls. For example, in the context of an optical communication system, a signal with higher baud rate may face impairments due to the optical filters. Also, wavelength selective switches, multiplexers, demultiplexers, and other network elements in an optical network often have limited bandwidth and a roll-off that may be too sharp to accommodate the higher baud rate.